Rechargeable, lightweight batteries are desired as miniaturized power sources for use in microelectronic devices in consumer, space, transportation and military applications. Solid state, high specific energy batteries are under development based on use of alkali metal-ionic polymeric complexes. Most of these batteries are based on lithium since it has such a low atomic weight and is highly electronegative. Lithium batteries based on liquid solvent containing electrolytes are unstable and present leakage problems. Molten electrolytes require high temperature environments.
The lifetime and number of discharge-charge cycles demanded of a lithium battery place stringent requirements on the components of the battery. The battery components also have mechanical, thermal and electrochemical constraints arising from the conditions of service. The electrolyte is subjected to the most stringent requirements since it provides several functions. The polymeric electrolyte must function as a lithium ion carrier, a separator and a binding agent for the positive electrode such as a titanium disulfide or cobalt oxide composite electrode. Solid state lithium-ion conducting polymeric electrolytes provide good adhesion, are deformable and have good elasticity.
Polyethylene oxide has been found to be the most satisfactory solid polyelectrolyte for use in thin film lithium batteries due to the formation of conducting complexes with lithium salts and the ability to be recharged. Batteries containing polyethylene oxide (PEO) films are compatible with other components of the battery and exhibit the mechanical properties required for a thin film battery. However, the PEO salt films exhibit low conductivity at room temperature and low cation transport.
Approaches to improving the conductivity of PEO-lithium salt electrolyte films include adding inert fillers such as Al.sub.2 O.sub.3 and plasticizers such as glyme to PEO or other polymer electrolytes (2,3) or mixing the PEO with another polymer such as propylene oxide (PPO), poly(bis-(methoxyethoxy ethoxide)-phosphazene) and the like (4).